Pumps having the following properties are known:                leak-tight closure of the ejection orifice so as to limit contacts between the outside air and the liquid being held inside the pump, in particular to prevent said liquid from drying out and/or degrading over time;        absence of contact between the liquid and metal parts, so as to prevent possible physicochemical degradations of the liquid;        absence of any passage for recirculation of air into the bottle in compensation for the restored product.        
The invention aims to simplify the production of such pumps by proposing a design consisting of parts that are simple to produce and limited in number.
Furthermore, the pump according to the invention proposes a sealing of the closure which is improved so as to be able to dispense liquids having a significant sensitivity to air. Consequently, the combined use of a pump according to the invention with a bottle containing an air-sensitive liquid is particularly advantageous.
In particular, anticipated as being sensitive to air are liquids containing a solvent likely to evaporate quickly, e.g., alcohol-based or water-based, or containing photosensitive substances, e.g., sunscreens, or easily oxidised, e.g., vitamins, particularly vitamin C.
In addition, the operation of the pump according to the invention limits the pressurisation of the liquid during dispensing. Thus, the combined use of a pump according to the invention with a bottle containing a liquid sensitive to mechanical stresses is also particularly advantageous.
In particular, anticipated as being sensitive to mechanical stresses are liquids, e.g., creams, likely to undergo a physicochemical transformation under pressure, in particular separation or a phase change.
The operation of the pump according to the invention also enables the dispensing of particularly viscous liquids.